The lock is stiff to operate with the door in the open position
We need to investigate the locking mechanism. Locks can become stiff to operate due to issues such as dirt in the lock cylinder, water ingress corroding the internal parts, lack of maintenance of the locking system, cold or hot weather causing the parts to cease or expand or finally the lock parts have simply broken through use.
If it’s the lock it could be either the lock barrel or the locking mechanism. To improve the lock barrel the best way to do this is with a graphite powder (we advise against WD 40 or a 3in1 type oil). A graphite powder can be found on online and can simply be blown into the lock cylinder and will improve the running of the key into the lock. A good hack here is to simply use a graphite pencil on the key and coating it in pencil lead.
One final check to make sure is to the cylinder fixing screw (the screw that holds the lock cyclinder to the door) isn’t too loose – if it is the screw head would be visibly proud of the lock gearing in this instance you can tighten the screw. Also it may be the screw is overtightened, this may require taking a phillips screw driver and slackening off half a turn.
If it’s the locking mechanism its important to check the running gear and lubricate it with a light engineering oil such as 3 in1. Any parts which are restricting movement should be checked to ensure that nothing is jamming the mechanism. Any loose parts can be re-tightened with a Philips head screwdriver.